1. Field of the Invention
The invention relates to cathode-ray tubes and, more particularly, to a device for the automatic modification of the cut-off voltage of a cathode-ray tube as a function of the luminance measured on the screen of the tube.
2. Description of the Prior Art
A cathode-ray tube 10 (FIG. 1) comprises in a chamber 11 under vacuum: a cathode 12 comprising a heated filament 16 that emits electrons and an anode 13 that is brought by means of a terminal 19 to a positive potential (HT) higher than the potential VK of the cathode so as to attract the electrons towards a surface 14 which constitutes the screen of the cathode-ray tube. The internal wall of the screen is coated with luminophores which get illuminated when they receive the electrons emitted by the cathode. This enables luminous images to be made to appear on the external wall of the screen by deflecting the path of the electrons, notably by means of variable magnetic fields created by deflection coils 15.
In order to achieve greater control over the path of the electrons and modulate the intensity of the electron beam, the electrons emitted by the cathode 12 go through a structure constituted by three electrodes or gates G1, G2 and G3 which are carried to potentials appropriate to their role. It is thus that the gate G1, better known as the Wehnelt gate, is positioned in the vicinity of the cathode and is at a negative potential VG1 with respect to this cathode so that it can stop or let through electrons going towards the screen. The gate G2, known as the acceleration electrode, is placed in the vicinity of the gate G1 towards the screen and is at a positive potential VG2 with respect to the cathode. Finally, the gate G3, known as the focusing gate, is placed before the deflection coils 15 and is at a positive potential VG3 with respect to the cathode.
In FIG. 1, the potentials of the different cathodes are obtained schematically by potentiometers 17, 18 and 101. The potentiometer 17 is connected between a terminal at +100 volts for example and a terminal connected to the ground. The potentiometer 18 is connected between the ground and a high voltage (HT) of 16 kilovolts for example. The potentiometer 101 is connected between the ground and a potential of -200 volts.
The cathode 12 is connected to the output terminal of the potentiometer 17 and its potential VK can therefore vary from 0 to +100 volts. The Wehnelt gate G1 is connected to the output terminal of the potentiometer 101 and its potential VG1 may therefore vary from 0 to -200 volts. The accelerator gate G2 is connected to a first output terminal of the potentiometer 18 and its potential VG2 may therefore vary from 0 to some thousands of volts. The focusing gate G2 is connected to a second output terminal of the potentiometer 18, and its potential VG3 may therefore reach several thousand volts.
It will be understood that the intensity of the electron beam and, hence, that of the luminous dot on the screen can be modulated by the modification of the voltage VGK1. To this effect, the gate G1 is biased at a voltage Vco, called a cut-off voltage, and a variable modulation voltage is applied to it to obtain a variable beam electron current and hence a variable luminance of the light dot on the screen.
The cut-off voltage Vco corresponds to the difference in potentials VKG1 which is just enough to prevent the passage of electrons towards the screen.
FIG. 2 is a graph showing the variation of the cathode current Ik which corresponds substantially to the luminance of the dot on the screen, as a function of the voltage VKG1 between the cathode and the gate G1. The curve 20, which is quasi-logarithmic, shows that the current Ik is zero for VKGl=Vco and that it reaches the value Iko for VKG1=0.
To obtain a linear characteristic between the signal applied to the gate G1 and the luminance on the screen, it is necessary, firstly, to linearize the curve 20 and, secondly, to hold the cathode-ray tube at its cut-off voltage in the absence of a modulation signal. This holding is all the more critical as the tube operates at low values of luminance, which it does when the cathode-ray tube is used in a dim environment.
To guarantee the stability of the low-level luminance, it is necessary:
always to bias the tube at its cut-off voltage;
to keep the voltage VKG2 stable between the cathode and the accelerator gate;
to keep the cathode heating power stable, i.e. ensure a certain precision and stability of the voltage Vf which is applied to the heating filament 16;
to keep the difference in potentials VKA between the cathode and the anode stable.
To solve these problems, it has been proposed to bias the tube with voltages VKG2, Vf and VKA that are as constant as possible, but it is difficult to maintain these voltages with a precision higher than 1%.
Furthermore, the characteristics of the tube, notably the cut-off voltage, change:
during the thermo-mechanical stabilization of the electron gun, when starting the system and
in the course of ageing during the life of the tube.
The result thereof is that the bias voltages would have to be readjusted in the course of time.
To compensate for these drifts, devices have been proposed for the servo-control of the cut-off voltage of the tube by the measurement of the cathode current. This servo-control is done at regular intervals, for example during the frame flyback or retrace of the image, and its value is memorized during the next frame.
The acquisition of the servo-control value is done in two steps:
a first step of applying, to the gate G1, a voltage greater than the cut-off voltage and of measuring the cathode leakage currents. The result of this measurement is subtracted from the measurement made in the second step and makes it possible to do away with the effects of the leakage currents;
a second step of applying, to the tube, a low modulation voltage of a known value and of servo-controlling the potential VKG1 so as to measure a cathode current Ik which is the sum of the leakage currents measured during the first step and of a constant current Iks corresponding to the value that would be generated by the desired value or set value of the modulation which is applied.
Such a method is satisfactory when the dynamic range of cathode current is between 10 micro-amperes and 2 milliamperes, which corresponds to servo-control currents Iks that are appropriate when the minimum light conditions are what are known as drawing room conditions as is the case with family television sets.
When the tube is placed in a very dim environment and/or when it is very sensitive (because of the high output of the luminophores), the servo-control should be done at cathode current values far lower than one microampere. This is difficult to achieve because of the values of the insulation resistance and of the inter-electrode parasitic capacitances.
Furthermore, this prior art method does not take account of the variation of the sensitivity of the luminophores, namely their light output, in the course of time.
Other devices have been proposed, using a photodetector placed before or optically coupled to a part of the screen that is not normally used. Because of the performance characteristics required of the screen, the single phosphor or luminophore that is used generally has a fairly lengthy response time of at least several milliseconds.
Consequently, if it is sought to obtain an electrical signal with an amplitude representing the luminance of the phosphore, it is necessary to maintain the test pulse for the same length of time.
For applications where it is acceptable to interrupt the display of the image temporarily to carry out the operation for the automatic correction of the cut-off voltage of the tube, the build-up time of the phosphor is not an inconvenient factor.
The particular feature of the invention is that it overcomes the problem of the build-up or raising time of the phosphor because any elimination, even a very occasional one, of a frame of the displayed image is considered to be inacceptable in many applications.